The present application relates to power semiconductor devices, and more particularly to rectifiers.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
One of the most basic elements of electric circuits is a rectifier, i.e. an element which will pass DC current in only one direction. (Since this is a two-terminal device, it is often referred to as a rectifying diode.) Such diodes are used in many motor control and power conversion circuits. In such applications, the currents which may have to be handled may be large.
Desirable features of a rectifying diode for power applications include low forward voltage drop, high reverse breakdown, fast reverse recovery, and low reverse leakage.
Junction Diode Rectifiers
As is well known, any p-n junction diode will have a voltage drop when current is flowing. For silicon, this is about 0.6 Volts. When large currents flow through a diode, a drop of even a volt will cause significant power dissipation. This power dissipation is undesirable.
Schottky Barrier Diode Rectifiers
A Schottky barrier diode (or “SBD”) will typically provide a lower forward voltage than a junction diode. However, Schottky diodes have more reverse leakage, and (at least in silicon) typically have lower reverse breakdown than junction diodes.
Field Effect Rectifiers
A device referred to as a field effect rectifier is described in U.S. Pat. No. 8,148,748. It has a structure similar to that of a lateral channel 1 DMOS FET, with its source and body, its gate, and its channel region located on the front surface, and its drain on the back surface. There are two major differences between the field effect rectifier of the '748 patent and a conventional DMOS FET. These differences are:
1. There is an additional p-type region in the center of the gate in the field effect rectifier. This additional p-type region or “p-pocket” is self-aligned to the edge of the source and body region that it faces. (This self-alignment is obtained by simultaneously forming the opening that is used to introduce dopant that forms these two p-type regions.) The distance between the p-intermediate region and the nearby p-doped body must be carefully controlled so the desired device characteristics are obtained.
2. The source and body, the gate, the channel, and the additional p-pocket region described above are all electrically connected to one terminal, with the drain region connected to the second terminal. (For a rectifier that uses with an n-type substrate, the metal region that connects to the first terminal forms the anode, while the metal region that connects to the drain forms the cathode.)
3. The body region's net dopant concentration as a function of location in the region between its source and its drain must be carefully tailored to provide the desired rectifying characteristics.
Processing steps that may be used include the use of an implanted body with a retrograde profile, the use of multiple implants, or the use of an oxidation step that depletes dopant from the surface region where the dopant has already been introduced. The goal of these processing steps is to obtain a body region with a net dopant that is either uniform or has its peak concentration closer to the “drain” end of the body. Either of these net body dopant profiles result in rectification.
The field effect rectifier described in the '748 patent provides a current vs. voltage curve with a lower forward voltage drop for the same current rating than both conventional pn-junction diodes and Schottky diodes. A more efficient rectifier allows electrical and electronic equipment to operate with lower power dissipation, which is an important and long standing goal.
Vertical Rectifier with Added Intermediate Region
The present application provides a new semiconductor rectifier structure. In general, a MOS-transistor-like structure is located above a JFET-like deeper structure. The present application teaches ways to combine and optimize these two structures in a merged device so that the resulting combined structure achieves both a low forward voltage and a high reverse breakdown voltage in a relatively small area.
In one class of innovative implementations, an insulated (or partially insulated) trench is used to define a vertical channel in a body region along the sidewall of a trench, so that majority carriers from a “source” region (typically n+) can flow through the channel An added “pocket” diffusion, of the same conductivity type as the body region (p-type in this example), is provided around the bottom of the trench. This intermediate diffusion, and an additional deep region of the same conductivity type, define a deep JFET-like device which is in series with the MOS channel portion of the diode. This advantageously permits the MOS channel portion to be reasonably short, and to have a reasonably low threshold voltage, since the high-voltage withstand characteristics are defined by the deep JFET-like device. The vertical nature of the carrier flow results in increased current density; this provides an area advantage as compared to other rectifier structures. The use of a single metallization on the top surface further increases density.
The pinch-off effect of the deep JFET-like device means that the MOS channel can be short and have a low threshold voltage, since the pinch-off effect will be sufficient to keep leakage low near reverse breakdown. Thus the MOS diode can be optimized for a low turn-on voltage, which provides a low forward voltage drop. For example, additional diffusion components can be added to reduce the turn-on voltage of the MOS diode, without degrading the breakdown voltage of the device.
Notionally, this device can be thought of as a series combination of a deep JFET-like depletion transistor with a MOS diode.
Because of the importance of the deep JFET-like device, the lateral separation between the intermediate diffusions and the additional deep regions has an important effect on the forward voltage drop of the device in operation.